If abusers terrorize without repercussion, then of course, victims have the right to kill abusers.

Abusers are so despicable that they are not human. They should be slaughtered like animals. Abusers torture people and manipulate others to agree with them. There are parents who abuse their children and husbands who abuse their wives. They all deserve to die. Sometimes, murder is the only logical way for a victim to end their suffering. Also, these monsters, mentally challenged or not, deserve nothing less painful than what they do to their victim. Our justice system should be changed to enable victims to kill abusers without repercussions. Victims are people to be sympathized with. Abusers, no matter what their issues are, deserved to be lynched and bashed for all eternity. When an abuser meets a well deserved demise, the world should not weep. Some victims are left traumatized for years after their abuser is out of the picture and have severe trust issues for the rest of their lives due to their experiences. Abusers are to be shunned and ridiculed, not cared for. Some abusers have been tried and got away scott free. The sooner the justice system changes to allow victims to eliminate their abusers if they choose to defend themselves, the sooner victims can eradicate more and more of these disgusting animals from being the world's problem.

If no one is doing anything about it...

Imagine a woman is being beaten by her husband, just as a stereotypical example, and he has completely isolated her from people so that she is not in a position emotionally, monetarily, or perhaps physically, to seek outside help (this is extraordinarily common in spousal abuse cases). One night he starts in on her for [insert cowardly, irrational reason to beat a woman here], and it shows no signs of stopping, reacting in such a way that ends up killing the abuser is completely rational. It's only human nature, basic survival instinct to "kill or be killed", which in normal society isn't right, but in extreme abuse cases is exactly what you would expect. Another instance might be the child who has been sexually abused and is emotionally and perhaps even mentally damaged to the point where they see no other option. Especially in sexual abuse cases, the victim is often so horribly ashamed of what has happened that they aren't capable of asking for help. When society fails to notice their pain, or fails to be able to prove it in a court of law, which is more often the case, their only option left is to surrender to the abuse or fight back. If you think running away is an option, you're right, but show me am abused twelve year old who thinks that's a realistic option and I'll show you a unicorn. I'm not saying murder is morally right, but the question was whether abuse constituted reason, and in some cases, when the rest of us fail the victim ("evil prevails when good men do nothing") it does.

It goes both ways

Victims of abuse/violence who kill their aggressors is an extremely grey area. From the perspective of the law, they committed a crime and must be punished, but from the viewpoint of the victim, murder was their only method to end the abuse. Take kids who kill their abusive parents and victims of domestic violence for example; often times the authorities do not make a difference and the victims are forced to deal with it on their own. And when that happens, they see no way out of it except to kill their abusers.

Murder is not the only way out.

While I can see why a victim would think this way, it's still not right. There are other solutions; telling a close friend or relative, going to the police to show them the bruises and scars you've suffered. Granted, none of these solutions are easy but they're less harmful than murder, especially to their psyche. When a person is physically or mentally abused their mind can't be thinking straight in the heat of an intense moment. The PTSD that arises from these horrible experiences can drive them to do things they would never normally do. Does the fact that they were abused excuse them? No, what they need is help.